The present invention relates to a pair of co-operating screw rotors, to a screw rotor, and to a rotary screw machine. Each rotor has helically extending lobes and intermediate grooves, through which the rotors intermesh, one rotor is a male rotor with each lobe in a section perpendicular to the rotor axes having a leading lobe flank and a trailing lobe flank, both being substantially convex. The other rotor is a female rotor with each lobe in said section having a leading and a trailing lobe flank, both being substantially concave. Each lobe of the male and female rotor has an asymmetric profile in said section.
In a rotary screw machine of the kind for which the rotors of the invention are intended, a compressible medium is compressed or expanded by intermeshing two rotors in a working space sealingly surrounding the pair of rotors which has the shape of two intersecting circular cylinders.
Decisive to the function and the efficiency of such a machine is the shape of its rotors, more precisely the shape of the flanks of the rotor lobes.
Normally, in a rotary screw compressor at work only one of the rotors is driving and transmits torque to the other one, the driven rotor. Usually a liquid is injected such as oil or water into the working space of the machine, which liquid forms a film on the flanks of the lobes for lubricating, cooling and sealing purposes. The lobes co-operate by intermeshing and are shaped to transmit torque between the rotors and to seal working chambers in the working space of the machine. An important aspect when designing the profiles of the lobes therefore is to attain a contact band between the rotors that in this respect is optimal. The contact band should be of sufficient size for the contact pressure which the material and the liquid film are exposed to. The contact band should have limits so defined that a minor error during manufacture such as pitch error, a wrong distance between the centres or a large deflection of the rotor does not have the consequence that the contact band will be materially displaced, which would lead to a risk for increased friction losses and breakdown. The shape of the profile should also be such that it allows the liquid to a high degree to gather on the surfaces of the contact band during periods when they are not in contact. The profile should in the areas neighbouring the contact band on both sides have a profile that does not move liquid from the surfaces of the contact band just before contact. Furthermore, the contact band should be well defined for measurement and control.
When designing the rotor profiles one has to take the total length of the contact band or the sealing line into consideration as well as other general aspects such as the size of the blow-hole, the contact forces, the volumetric capacity, thermal expansion, generation of vibrations and demands relating to the manufacture. There are also some mathematical limitations for the profiles. For some compressors, certain aspects are more important than others and for other compressors there might be reasons to give priority to other aspects. An optimal profile usually represents a compromise between different requirements related to these aspects, the compromise being dependent on which of these are the most important in the actual case.
Due to the decisive importance of the shape of the rotor profiles in a rotary screw machine and due to the complex weightening between the aspects that have to be considered there are a large number of granted patents focusing on the profiles, all since Lysholm during the thirties presented and got a patent for the first rotary screw compressor of this kind that could be used in practice.
There are many ways in patent literature in which the rotor profiles are defined, depending on which problem(s) the patent relates to and due to the complicated shape of these profiles. The profiles are thus defined as a family of characteristics, a combination of such, by some important parameters, by ranges for certain features of the profile, by expressions implicitly defining the profile or in another way. Furthermore the profiles can be divided into different categories according to various criteria such as symmetric or asymmetric profiles or such as circular, point generated or travelling-point generated.
The present invention primarily is related to attain a profile with an optimal design of the contact band taking those of the above requirements into consideration that should be relevant for that.
A principal problem to be solved is that the width of the contact band has to be limited since a minor error in manufacture otherwise would displace the contact band to areas where the relative speed between the contact surfaces is large and the allowable surface pressure of the material is exceeded. This would mean a decreased efficiency and a risk for break-down due to surface rupture in the material.
Since furthermore the extension of the contact band is substantially longitudinal and the end positions in the plane are very hard to define in practice there will be great difficulties to measure and evaluate the manufactured rotor. This means high costs for measuring the profiles.
Furthermore the gradual change in the plane and along the contact band from non-contact to contact results in that the neighbouring surfaces remove the liquid layer from these surfaces that later enter into the contact band. This means less effective lubrication resulting in wear and vibrations.
These problems entail all point generated and travelling-point generated profiles except the symmetric profiles. A symmetric profile however suffers from other drawbacks such as bad efficiency etc.
When the mentioned problem have occurred one usually has taken various measures to solve them. When the material stress has been too high it is possible to use harder or hardened material, which is expensive. It has also been developed profiles with smaller transmitted torque and larger travelling-point generated contact bands. This has increased the risk for vibrations since the contact sometimes is lost due to torque pulses. In addition one faces the risk of contact on profile portions where contact is undesired due to manufacturing error or rotor deflection.
Through European patent No. 149 304 it is earlier known a pair of rotors of the kind in question where one has tried to attain an improved shape of the rotor lobe flank profiles, whereby in particular the shape of the contact band has been focused on. Thus it is disclosed a contact band formed by a concave circular arc on the leading female rotor flank and a complementary convex circular arc on the trailing male rotor flank co-operating therewith. This construction of the profile has the disadvantage that it can be used only for female drive compressors. Furthermore, the circular arcs according to this construction have the centre defined to the rolling point, i.e. the rolling angle into contact=0.
The object of the present invention is in this context to attain rotors with lobe profiles that solve the problems mentioned above without having the drawbacks entailing previous attempts to attain this.